The field of this invention is fluid compensation devices and in particular fluid compensation devices compensating for drilling fluid flow changes in an offshore floating drilling platform fluid return system caused by wave and tide action.
A typical offshore oil well drilling system may include a floating drilling platform which floats on the surface of the water. The floating platform is attached by a telescopic connection to a marine riser which extends downwardly to the ocean floor. The drill string, which is supported on the floating drilling platform and rotated therefrom, is housed within the marine riser. Drilling fluid is circulated downwardly through the drill string from the platform, out the drill bit end and then upwardly in the annular space between the drill string and the marine riser. The telescopic connection between the floating oil well drilling platform and the marine riser is formed by a telescoping cylindrical member which is attached to the floating platform and is mounted for slidable, sealable movement within the top of the marine riser. The telescoping cylindrical member is connected to a drilling fluid return line at an opening in the cylindrical member in order to transfer the upward flowing drilling fluid back into the well fluid system.
Since the oil well drilling platform floats on the surface, the platform is subject to vertical movement or heave as a result of wave and tide action. Vertical movement of the platform causes similar vertical movement of the telescoping member within the marine riser. Movement of the telescoping member downwardly causes an increase in flow space consumed by the drilling fluid above the return opening in the telescoping member, which precipitates a temporary increase in fluid flow in the return line. Further, movement of the telescoping cylindrical member downwardly decreases the available flow space within the marine riser, which also causes an increase in drilling fluid flow. Conversely, upward heave of the platform causes upward movement of the telescoping cylindrical member within the marine riser. This causes a temporary loss or reduction in drilling fluid flow space consumed in the telescoping member. Further, upward movement of the telescoping connection member results in an increase in available flow space within the marine riser and a further decrease or loss in the drilling fluid flow space consumed in the telescoping member. Such increases or decreases in the volume of fluid flowing upwardly from the drill string as a result of heave of the platform complicate the problem of monitoring the return fluid flow rate for the detection of problems downhole. One possible solution to this problem is found in U.S. Pat. No. 3,910,110 of Jefferies et al. The Jefferies patent discloses a system for detecting the commencement of a blowout or lost circulation in a sub-aqueous well being drilled from a floating vessel in which drilling fluid is being used. The rates of flow of the drilling fluid into and out of the well are monitored, compared and a signal proportional to the difference therebetween is produced. The electric signal is modified to compensate for change in the volume of the flow of the drilling fluid caused by the heaving motion of the vessel. Similarly, U.S. Pat. No. 3,976,148 provides a measurement system for measuring variables caused by movement of the telescoping joint mounted in the marine riser and for correlating electrical signals measuring such variables in order to produce a signal proportional to the flow rate of the return drilling fluid flowing in the marine riser annulus. Other patents which may be of interest include U.S. Pat. Nos. 3,760,891; 3,911,740; 3,917,006; 3,905,580; and 3,889,747.